1. Field of the Invention
The subject disclosure relates to fuel cells, and more particularly to solid oxide fuel cell systems having improved hot zones and tail gas combustors.
2. Background of the Related Art
Referring to FIG. 1, a schematic view of a general solid oxide fuel cell (SOFC) 110 with a hot zone 112 is shown. The SOFC 110 has an electrochemical stack 120 typically operating at temperatures above 700° C. Several other support components are also operating at elevated temperature. The area of integration of the elevated temperature components is referred to as the hot zone 112. The hot zone 112 is insulated to reduce heat loss and maintain the desired operating temperature.
The components of the hot zone 112 include a catalytic partial oxidation (CPOX) reactor 114 for converting the system feed hydrocarbon fuel to a hydrogen and carbon monoxide rich feed for the stack 120 of the SOFC 110. A tail gas combustor 116 burns the remaining unutilized fuel from the stack 112 to reduce CO emissions and also to aid in other endothermic reactions. A recuperator heat exchanger 118 decreases the SOFC exhaust temperature by cooling the exhaust gas with the inlet stack air. By heating the inlet stack air and using a vaporizer 122, the feed hydrocarbon fuel is readied for the CPOX reactor 114.
A power conditioning unit 124 also connects to the electrochemical stack 120. A blower 126 provides air to the CPOX reactor 114. A fuel tank 128 and fuel pump 130 provide fuel to the vaporizer 122. A cathode blower 132 provides air to the recuperator 118.
The approach of FIG. 1 has several disadvantages. Each component requires piping to connect to the neighboring component. This plumbing requires a high temperature sealing method such as brazing or welding, a very labor intensive and extremely difficult to automate process. Each hot zone component also requires special features for braze or weld joints to the attached piping. These special features are typically machined, and result in high cost components.
Further, some stack components are ceramic, and sealing metal-to-ceramic joints is difficult such as shown in U.S. Patent Application Publication No. 2004/0195782 published on Oct. 7, 2004. The hot CPOX reactor 114 and tail gas combustor 116 are located away from the electrochemical stack 120, which slows heating at startup. Assembling and joining these components in close proximity is difficult and requires packing a large volume in a small space. Consequently, the support components can undesirably occupy as much hot zone volume as the electrochemical stack 120. The inability to closely integrate the hot zone components leads to a low hot zone power density.
For indoor applications, high fuel utilization is particularly desirable for efficiency and proper emissions. The SOFC 110 cannot alone utilize the fuel unless the SOFC 110 is impracticably large. Thus, the performance of the tail gas combustor 116 is particularly important and serves as a source of thermal energy that can be used for other needs. For instance, the thermal energy can keep the stack 120 at operating temperature and balance heat losses through insulation and system exhaust.
The tail gas combustor 116 is typically catalytic or homogeneous in nature. Catalytic combustors have the advantage of being able to operate over a wider temperature and concentration range than homogeneous flame combustors. A SOFC stack 112 operated at about 75% utilization that is fed products from the CPOX reactor 114 will output a tail gas that might experience a 300 degree temperature rise in an adiabatic tail gas combustor. For a stack 112 operating at 800° C., this would result in a tail gas combustor temperature of approximately 1100° C. Real tail gas combustors operate non-adiabatically, and measured temperatures for a tail gas combustor are typically at least 900° C. In view of the above, the tail gas combustor 116 is made to perform optimally under high temperatures. However, at the low temperatures of start up, these tail gas combustors 116 perform poorly and emissions suffer.
Additionally, the tail gas combustor 116 may be poorly suited to temperature extremes and even breakdown during exposure. For example, one catalyst used for the tail gas combustor 116 is a conventional noble metal catalyst such as platinum. The high activity of the platinum ensures quick light-off in a cold system and enables a very compact tail gas combustor 116. But, one significant disadvantage of noble metal combustion catalysts is their low stability in high temperature environments. Metal loss due to evaporation eventually leads to decreased activity and poor combustor performance. While sustained operation at 1100° C. is atypical, higher temperatures can lead to rapid catalyst evaporation and metal depletion in minutes. These conditions can occur easily in off-design conditions, such as when a SOFC operating at steady state suddenly experiences an open circuit condition by the user disconnecting the system load. A sudden open circuit condition, where no current is drawn by the SOFC stack 120, will reduce utilization to zero. Thus, all of the fuel and cathode air will be combusted in the tail gas combustor 116. These gases can experience over a 1000° C. adiabatic temperature rise, which when entering the tail gas combustor 116 at 800° C. can result in an elevated tail gas combustor temperature as high as 1800° C. This temperature is certainly high enough to destroy a noble metal catalyst quickly.
Some steps can be taken to minimize the temperature effects of off-design conditions on the tail gas combustor 116. For instance, a protection circuit can be used to continue to draw current from the stack 120 if the system load is disconnected. A more robust tail gas combustor would help alleviate such concerns. One option is a metal oxide combustion catalyst that is more stable than the noble metal materials. Perovskite catalysts are one example. While such materials are more stable and able to operate in very high temperature environments for longer periods, their activity is much lower than noble metals. Hence, performance suffers. Indeed, even when ignited, a low activity combustion catalyst has difficulty completely burning all of the carbon monoxide in the system exhaust.